1 Field of the Invention
This invention relates to an exposure method of a semiconductor wafer by a mercury-vapor lamp.
2 Description of the Prior Art
Upon fabrication of a semiconductor device such as an integrated circuit, large-scale integrated circuit, super large-scale integrated circuit or the like, a photofabrication process is carried out. For example, in order to remove portions of a silicon oxide film formed on a surface of a substrate, which is for example a silicon wafer, a photofabrication process is carried out in accordance with an image pattern such as a circuit pattern. This photofabrication process includes such steps that a photoresist film is formed over the silicon oxide film on the silicon substrate and the photoresist film is then exposed to ultraviolet rays through a photomask having a pattern image. After exposure, the photoresist film is developed and the silicon oxide film is then subjected to an etching treatment. Thereafter, a circuit-forming treatment such as diffusion, ion implantation or the like is applied to the silicon substrate through the thusetched silicon oxide film.
A semiconductor wafer is generally circular with its surface area supposed to be divided into minute square sections arranged rows and columns. These minute sections will each be cut afterwards to form chips which will be semiconductor devices respectively. A sheet of semiconductor wafer is generally 3 inches, 5 inches or 6 inches across. The sizes of such semiconductor wafers tend to increase coupled with progresses in their fabrication technology.
A high-output mercury-vapor lamp is indispensable in order to expose the entire surface of a semiconductor wafer simultaneously so that all the minute sections, which will individually be formed into chips, are printed at once. Use of such a high-output mercury-vapor lamp is however accompanied by such problems that it renders an exposure system in which the lamp is assembled large and a considerably high degree of technique is required for the uniformity of illuminance on the surface of the semiconductor wafer. Consequently, it is very difficult to meet the tendency of enlargement of semiconductor wafers practically.
With the foregoing in view, it has recently been proposed to expose minute sections, which are arranged in rows and columns on a semiconductor wafer, one after another successively so that pattern images are printed successively and respectively on the minute sections. In such a stepwise exposure method, it is carried out to expose an area equivalent to only one of the minute sections in each exposure operation. Therefore, the stepwise exposure method permits the use of a low-output mercury-lamp, whereby bringing about such substantial advantages that an exposure system employed would have been reduced in size and the illuminance can be readily made uniform on the surface of each semiconductor wafer because the area of each exposure is small. As a result, a pattern image can be printed with a high degree of accuracy.
A mercury-vapor lamp cannot however be repeatedly turned on and off in a short cycle because the enclosed mercury vapor undergoes condensation while the lamp is turned off. It is therefore advantageous to cause a mercury-vapor lamp to light repeatedly and alternately at a low power consumption level and at a high power consumption level while maintaining the mercury-vapor lamp in a continuously-lit state, to expose a minute section of a semiconductor wafer, which minute section has assumed an exposure position, to the light from the mercury-vapor lamp when the mercury-vapor lamp is lit at the high power consumption level, and when the mercury-vapor lamp is lit at the low power consumption level, to shift the semiconductor wafer stepwise so that another minute section of the semiconductor wafer, which another minute section is to be subjected to next exposure, is allowed to assume the exposure position, while the light from the mercury-vapor lamp is cut off by means of a shutter. The above manner can provide a required level of light quantity at the high power consumption level and at the same time, keeps the mercury-vapor lamp in its lit state at the low power consumption level while avoiding wasting of electric power.
In such a stepwise exposure method, the light of the mercury-vapor lamp is not utilized while the shutter is closed, resulting in such drawbacks that lots of electricity are still wasted and the shutter is susceptible to considerable damages due to its exposure to the high-energy light. The shutter is required to operate quick, because if its opening or closing motion should be slow, non-uniform exposure of a semiconductor wafer due to such a slow opening or closing motion of the shutter becomes a problem. In order to meet this requirement, it is indispensable that the shutter has a light weight. However, a light-weight shutter will inevitably result in poor heat resistance. As a result, such a light-weight shutter tends to undergo deformation due to heat which is built up while shielding the light and hence to develop a malfunction which impairs its smooth opening and closing operation.
With the foregoing in view, it may be contemplated to light, during each closure period of the shutter, the mercury-vapor lamp with a power consumption smaller than its power consumption during the exposure time, i.e., while the shutter is kept open.
It has however been found that the above exposure method is accompanied by a new problem. Namely, it becomes increasingly preferable from the viewpoint of increasing the quantity of light to be radiated as the amount of filled mercury increases, because the quantity of light to be radiated from a mercury-vapor lamp increases corresponding to the amount of mercury to be filled in its envelope. On the other hand, it is necessary that mercury vapor does not condense even when the mercury-vapor lamp is lit with smaller power consumption in each non-exposure period in which the shutter is closed. If the amount of mercury to be filled is set in such a way that the vapor pressure of mercury in the envelope when the mercury-vapor lamp is lit at small power consumption will become close to the saturated vapor pressure of mercury at that time, the mercury-vapor lamp will be fallen into an incomplete lighting state in an instant when the power consumption of the mercury-vapor lamp is changed to a higher level. Thus, the quantity of the radiated light will not increase immediately and the degree of exposure will probably be insufficient.